1. Background of the Invention
This invention relates to a printhead used in equipment for forming black and colour images, by way of successive scanning passes, on a print medium, normally though not exclusively a sheet of paper, using the thermal type ink jet technology, and more particularly to the actuator assembly of the head, and to the relative manufacturing process.
2. Prior Art
The composition and general mode of operation of a printhead according to the thermal type technology, and of the xe2x80x9ctop-shooterxe2x80x9d type in particular, i.e. those that emit the ink droplets in a director perpendicular to the actuator assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on only some of the features of the heads and their manufacturing process, or relevance for the purposes of understanding this invention.
FIG. 1 shows an enlarged perspective view of an actuator assembly 80 of a monochromatic ink jet printhead, consisting of a die 51 of a semiconductor material (usually Silicon) on the upper face of which resistors 52 have been made for the emission of the ink droplets, driving circuits 53 for controlling the resistors 52, pads 54 for connecting the head to an electronic controller, not depicted in the figures, a resistive temperature sensor 65, reference marks 69, and which has a pass-through slot 55 along which the ink flows from a tank not shown in the figure. Attached to the upper face of the die is a layer 60 of photopolymer having a thickness less than or equal to 25 mm wherein are made, using known photolithographic techniques, a plurality of ducts 57 and a plurality of chambers 64 positioned in correspondence with the resistors 52.
Stuck above the photopolymer 60 is a nozzles plate 61, usually made from a sheet of gold-plated Nickel or of Kapton, of thickness 50 xcexcm or less, bearing a plurality of nozzles 62, each nozzle 62 being in correspondence with a chamber 64. In the current art, diameter of the nozzles is usually between 10 and 60 xcexcm, while their centers are usually set apart by a step A of {fraction (1/150)} or {fraction (1/300)} of an inch (169 xcexcm or 84.5 xcexcm). Usually, though not always, the nozzles 62 are disposed in two parallel rows, staggered by a distance B=A/2, in order to double the resolution of the image in the head scanning direction, which accordingly becomes {fraction (1/300)} or {fraction (1/600)} or an inch.
Also in FIG. 1 the axes x, y and z giving the three-dimensional references of the die 51 are defined.
The traditional process for manufacture of the actuator assembly will now be described below in brief, with reference to the flow diagram of FIG. 3, starting from a first step 70 in which a wafer 66 is made available whereupon the dice 51 are made (FIG. 2). In a subsequent step 71, the wafer 66 is tested. In a step 72, the wafer 66 is coated with a layer of photopolymer, generally of the dry film type.
In a step 73 the photopolymer is exposed and, in a subsequent step 74, the chambers 64, in line with the resistors 52, and the ducts 57 are made in the layer of photopolymer 60 (FIG. 1), through development using known techniques.
In a step 75 a protection is applied to the entire wafer and, in a subsequent step 76, the slots 55, which bring the ink to the ducts 57, are cut by way of a sandblasting operation. In a step 77, the protection is washed off and a sight check is made that the component is still whole.
In a subsequent step 100, the nozzles plates 61 are positioned in such a way that the nozzles 62 are aligned with the chambers 64, and stuck on the dice 51 belonging to the wafer 66. Subsequently (step 101 ) the wafer 66 is applied to an adhesive tape 113 (FIG. 4), mounted on a frame 114. The individual dice 51 are separated in a step 102 by cutting with a diamond wheel 115, 50÷100 mm thick (FIG. 5), but are kept fast in their original positions by way of the adhesive tape 113 to which they adhere. Washing and drying are then performed (step 103), using an Ultratech machine for example.
In a step 105, a pick and place device of known technology, picks each die 51 off the adhesive tape 113 and places it with precision (error less than xc2x110 xcexcm on the x axis) on an alignment base. In a step 104, in the form of a continuous reel, a multiplicity of flat cables 117 (FIG. 1) is supplied separately, each having a window 122 with fingers 123 that will be soldered to the connecting pads 54 of the dice 51, machine contacts pads 121 and interconnecting tracks 120 which connect the pads 121 to the fingers 123. IN a step 107 the flat cable 117 is aligned with the die 51, with a tolerance of xc2x15 xcexcm on the x and y axes. In a step 110 an ultrasound soldering head comes into position above the connecting pads 54 of the die 51, whereto it solders one by one all the fingers 123 of the flat cable 117 (point-to-point TAB). The operations involved in the steps 105, 107 and 110 are effected using the technique known as Tape Automatic TAB).
In a subsequent step 111 the individual flat cables 117 are separated into distinct actuator assemblies 80.
A variant of the known art consists in making the nozzles directly on the flat cable (U.S. Pat. No. 5,278,584), which accordingly also has the function of nozzles plate, and is illustrated in FIG. 6. The flat cable 180 with nozzles is applied on a die 183 in which the feeding of the ink is effected from both sides. As a result, the windows 181 containing the fingers 123 are disposed perpendicularly to the ends of the rows of nozzles.
As the technology evolves, so the demand grows for heads with an ever greater number of nozzles, in order to reduce the number of scanning passes the head needs to complete a page and improve the printer""s productivity. To increase the number of nozzles, dice must be produced that are longer and longer and have the minimum possible width (4÷5 mm, where the mechanical requirements permit) so as to better exploit the wafer 66.
Accordingly the slots 55 are particularly long (typically though not exclusively greater than 12.5 mm) and are an open invitation for the dice 51 to break. When the nozzles plates (step 100) are assembled conventionally, the risk of the entire wafer 66 breaking when under pressure during soldering is high, with considerable economic damage.
Even when the step 100 is completed without damage, there is still a high risk of the individual dice 51 breaking in the subsequent machining operations, with serious economic damage on account of the notable dimensions of the dice 51 themselves. With a step A (see FIG. 1) of less than {fraction (1/300)} of an inch, in practice the nozzles plate have to be produced in kapton. This further increases the risk of the dice 51 breaking.
The object of this invention is to solve the problem represented by the risk of the dice breaking during the different machining stages of the nozzles assembly of an ink jet printhead, whether monochromatic or colour, by sticking the wafer on a rigid substrate and, instead of cutting the slot in a sandblasting operation, by effecting instead a through cut over the entire length of the dice.
Another object is to handle the individual dice, rendered fragile by the slot, with safety and not expose them to the risks of breaking, keeping them stuck upon a portion of the said base.
A further object is to make resistors underneath said substrate such that the operation of soldering the nozzles plates on the dice may be effected more rapidly, with local heating and a soldering temperature controlled by a sensor.
A further object is to improve the thermal dissipation of said actuator, by using the contribution to heat conduction made by said substrate.
A further object is to lower the time to refill the chamber following emission of the droplet of ink, since the edge of the through cut made with a diamond wheel, is more precise than the edge of the slot made by sandblasting, and can therefore be made at a lesser distance from the resistors.
The above objects are obtained by means of an ink jet printhead with a large-size Silicon wafer and relative manufacturing process, characterized as defined in the main claims.